(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of creating an attenuating rim type phase shift mask.
(2) Description of the Prior Art
One of the major technical disciplines that is used for the creation of semiconductor devices is photolithography, which is frequently and commonly used for the creation of patterns of various designs in layers of semiconductor materials or over semiconductor surfaces. A light source, commonly comprising Ultra Violet or Deep Ultra Violate energy, is used to project an image from a photolithographic mask onto a target surface, such as the surface of a layer of photoresist. The mask that is used between the light source and the target surface is composed of regions of light transparency and regions that completely or partially block light from passing through the mask. These characteristics of the photolithographic mask result in energy being transferred from the light source into the target surface, a transfer that changes chemical and molecular properties of the layer that forms the target surface. Photolithography provides a system of one or more optical lenses through which the light passes before striking the target surface. These lenses have as design objective to create the perfect reflection of the image that is contained in the photolithographic mask onto the exposed surface. Light of the energy source is transmitted to the target surface as waves of a particular frequency. Amplitude and phase of the projected light can, in its passage from source to target, be affected in accordance with the laws of physics. Ideally all the light that strikes the target surface does so under the exact same conditions of frequency, amplitude, phase and angle of impact so that the image that is created in the target surface is uniform across that surface. A serious problem in this respect is the effect of optical diffraction, whereby the light that impacts a surface does so under an angle that varies across the target surface. This effect becomes particularly severe where devices are created of micron or sub-micron dimensions, whereby any deviation from an ideal geometry of the created device features has a relatively large impact. Reduced device feature size also brings with it requirements of improved image resolution since the adjacency between device features across the surface of a semiconductor surface decreases with decreasing device feature size. The resolution of the created image on a target surface is essentially determined by the optimum available numerical aperture of the lens system that is used for the image formation. Improving this performance parameter however is in conflict with the desire to achieve optimum depth of focus of the exposed image, since the depth of focus of a lens system is inversely proportional with the numerical aperture of the lens system. To provide the required ideal image over a target surface, a number of corrective measures can be used that offset the undesirable characteristics of the system that is used to create this image. These corrective measures can make use of one or more of the parameters that play a role in the creation of the ideal image such as light amplitude, frequency and phase. In addition, the method in which the image is created can be changed by for instance “pre-distorting” the image, that is by creating an image before the exposure occurs. By taking into account the distorting effects of the image formation process, an ideal image or an image that closely approaches an ideal image can be formed on the target surface in this manner. It is clear that these corrective measures are highly image dependent in the sense that, across the surface of a relatively large wafer, effects that are applied to the center of the wafer surface may produce entirely different results at the perimeter of the wafer.
One of the frequently applied techniques for the creation of device features that are in close physical proximity is the use of a phase-shift mask of which the attenuating phase shift mask is one application. The invention provides a process of creating a rim type attenuating phase shift mask, whereby the emphasis is placed on providing a simplified and therefore very cost-effective method for the creation of the mask.
U.S. Pat. No. 5,620,817 (Hsu et al.) shows a method for the creation of an Attenuating PSM.
U.S. Pat. No. 5,955,222 (Hibbs et al.) reveals a Rim type PSM and method al.
U.S. Pat. No. 5,300,379 (Dao et al.) shows an inverted PSM.
U.S. Pat. No. 6,007,324 (Tzu et al.) and U.S. Pat. No. 61,294,295 B1 (Lin et al.) are related mask patents.